A Leonid Fireball Meteor from 1966.
Image: J. W. Young
Source: TMO, JPL, NASA.
Question: I saw a very bright object moving across the sky last night. It seemed to be right above us and was travelling quite fast. Can you tell me what it was?
Answer: The object you saw last night was almost certainly a meteor (shooting star).
The only other objects that move noticeably across the sky (other than aircraft) are satellites. Satelites move considerably more slowly than meteors and are only visible just after dark or just before dawn.
A very bright meteor (brighter than any of the stars or planets) is called a fireball. Sometimes a fireball is also called a bolide although often the name bolide is used only for large meteors that explode at the end of their flight.
It is very difficult to judge the distance to such small, fast moving objects, so most people imagine meteors to be closer than they really are. Meteors lose visibility about 20km above the ground. Thus, if you could still see the meteor, it would have been at least 20km away.
Most meteors (and around one in three fireballs) are fragments of comets made up of ice and dust. They are generally very small, about the size of a grain of sand. Meteors this size usually burn up completely about 100km from the Earth’s surface, generating a huge amount of light as they do so.
A small number of meteors (but a much larger proportion of fireballs) come from rocky asteroid material. Most are small (about the size of a pebble) but they can be up to kilometres across. Large rocky meteors do not always burn up completely. Meteors that survive the passage through the Earth’s atmosphere and hit the ground are called meteorites.
Two specimens of the Murchison meteorite which fell at Murchison, 80 km north of Melbourne, on the 29 September 1969.
Photographer: Frank Coffa / Source: Museum Victoria
Reports of observations of meteors are very useful to astronomers. People who do sight these events are encouraged to report them to the Astronomical Society of Victoria Skyline (03 9888 7130) or the International Meteor Organisation.